The present invention relates to electrical inspection systems and methods and, more particularly, to a systems and methods for the non-contact testing of electrical conductors, such as those found on printed circuit boards (PCB's), liquid crystal devices (LCD's), multi chip modules (MCM's) or integrated circuit devices for electrical continuity or shorts.
Currently known methods for electrical testing of small conductors can be broadly classified into methods which require physical contact of one or more probe with the conductor being tested and methods which do not require such physical contact.
Among the methods which require physical contact are the methods known as `flying probes` and `bed of nails`. The `flying probes` method use metallic probes mounted on an X-Y positioning system. The probes are made to touch a conductor at two points and a potential difference is introduced between the two probes. The potential difference brings about a measurable current flow through the conductor, provided that the conductor is electrically continuous. Inversely, the probes can be made to touch different conductors. In this case a measurable current flow indicates a short circuit between the two ostensibly separate conductors.
In the `bed of nails` method a matrix of probes is made to contact the printed circuit board. Electrical continuity and/or short circuiting is then monitored through the proper imposition of potential differences between pairs of nails and the simultaneous monitoring of any current through the nails.
Electrical testing methods which rely on physical contact of the probes with the conductors being inspected suffer from at least two basic disadvantages. First, as the size of conductors continues to decrease it becomes harder and harder to effect adequate physical contact, and thus electrical contact, between the probes and the conductor. Second, the physical contact between the probes and the conductors, especially the very small conductors, can often lead to damage of the conductors in the testing process.
To overcome these difficulties, a number of non-contact electrical testing methods have been proposed. Several proposed non-contact methods make use of an electrically-conductive laser-induced plasma which bridges the gap between a probe and the conductor to as to close an electrical circuit between the probes and the conductor without direct physical contact between the probes and the conductor.
U.S. Pat. No. 4,970,461, which is incorporated by reference, discloses placing the circuit board in a gas-filled chamber. A laser is directed to a grid located above the electronic circuit board. The laser induces a discharge from the grid across the field which causes the building up of charge in the node. When the field is reversed a visible plasma is created as the charged node is charged which can be visually observed providing an indication of electrical continuity.
U.S. Pat. No. 5,017,863, which is incorporated by reference, uses a laser beam to impinge on a photoemissive grid located close to the printed circuit board being inspected. Electrons emitted from the grid serve to close the circuit and provide electrical continuity information.
Finally, U.S. Pat. No. 5,202,623, which is incorporated by reference, generates a laser-activated plasma in a small chamber which is subjected to a concentrated laser pulse. The generated plasma exits through an orifice in the chamber and the electrically-conducive plume of ionized gas is used to close a circuit.
A difficulty shared by each of the non-contact electrical inspection methods described above is their inability to accurately test very small conductors.
There is thus a widely recognized need for, and it would be highly advantageous to have, a simple and reliable system and method for the non-contact electrical inspection of conductors which is convenient and accurate in the inspection of even very small features of printed circuit boards, integrated circuits, multi chip modules liquid crystal devices, and the like.